Light emitting diode device with luminescent material

ABSTRACT

The invention provides a light emitting diode device comprising a light emitting diode ( 1, 11 ) arranged on a substrate ( 2, 12 ) and a wavelength converting element ( 3, 13, 14 ). The wavelength converting element ( 3, 13 ) contains as a luminescent material a Mn 4+ -activated fluoride compound having a garnet-type crystal structure. The Mn 4+ -activated fluoride compound preferably answers the general formula {A 3 }[B 2-x-y Mn x Mg y ](Li 3 )F 12-d O d , in which formula A stands for at least one element selected from the series consisting of Na +  and K +  and B stands for at least one element selected from the series consisting of Al 3+ , B 3+ , Sc 3+ , Fe 3+ , Cr 3+ , Ti 4+  and In 3+ , and in which formula x ranges between 0.02 and 0.2, y ranges between 0.0 (and incl. 0.0) and 0.4 and d ranges between 0 (and incl. 0) and 1. Said compound is most preferably {Na 3 }[Al 2-x-y Mn x Mg y ](Li 3 )F 12-d O d . The invention also provides said material as well as a method for its preparation. As the luminescent materials of the described type and structure have high stability and low sensitivity towards humid environments, they can advantageously be used as in wavelength conversion elements of LED devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 13/511,159, filedMay 22, 2012, which claims priority to EP 09179553.4 filed Dec. 17,2009. All previously filed applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The invention relates to a light emitting diode device comprising alight emitting diode arranged on a substrate, and a wavelengthconverting element containing a Mn⁴⁺-activated fluoride compound. Theinvention also relates to a luminescent material as well as to a methodfor preparing such luminescent material.

Light emitting diode devices (abbr. as LED devices) are widely known asnew semiconductor light sources with promising lighting properties forfuture applications. These LED devices should eventually substitute manyof the current light sources, like incandescent lamps. They areespecially useful in display lights, warning lights, indicator lightsand decoration lights.

The color of the emitted light depends on the type of semiconductormaterial. LEDs produced from Group III-V alloys—such as GaN—arewell-known for their ability to produce emission in the green to UVrange of the electromagnetic spectrum. During the last decade, methodshave been developed to convert (parts of) the radiation emitted by such‘blue’ or ‘(near)UV’ LEDs into radiation of longer wavelength. Phosphorsare widely used luminescent materials for this purpose. These phosphorsare crystalline, inorganic compounds of high chemical purity andprecisely controlled compositions. They comprise small amounts ofspecifically selected elements ('activators'), which make them toefficient luminescent materials.

In addition to colored LEDs, the development of so-called ‘white lightLEDs’ is also very important. An interesting configuration in this fieldis based on converting a part of the light generated by a blue/UV LEDand mixing that converted part with the non-converted part of saidgenerated light, so obtaining white or white-like light. In this areablue emitting GaInN LEDs are most popular. Ce³⁺-activated YttriumAluminum Garnet (YAG-Ce) and Eu²⁺-activated Ortho Silicates (BOSE, OSE)are well-known phosphors for this purpose.

A LED device as described in the opening paragraph is known as such, forexample from the patent publication WO 2009/012301-A2. This documentdescribes in great detail a number of LED devices in whichMn⁴⁺-activated fluoride compounds are applied as a luminescent materialin the wavelength converting elements of these devices. Emission andexcitation spectra of a number of K₂[XF₆]:Mn⁴⁺ (X=Nb or Ta) andK₃[XF₇]:Mn⁴⁺ (X=Bi, Y, La or Gd) phosphor compounds are shown. Theseluminescent materials appear to show a narrow band or line emission inthe red spectral region (600-660 nm) of the electromagnetic spectrum.This is very attractive as LED devices comprising such luminescentmaterials in their wavelength converting elements are able to produce‘warm white’ light. This is light with a comparative color temperature(CTT) below 5000K.

OBJECTS AND SUMMARY OF THE INVENTION

The known LED devices have several disadvantages. A first disadvantageto be mentioned concerns the fluoride compounds used in the wavelengthconverting elements, most of which are (less or more) toxic. A seconddisadvantage pertains to the handling of these fluoride compounds, whichin practice is not easy, due to their relatively high sensitivitytowards humid environments. Prolonged exposure of these materials to(humid) air causes formation of a thin water film on the surface of thematerial, leading to (surface) decomposition. This disadvantageousproperty affects both the pure materials (causing short shelf times) andthe LED devices in which they are applied (causing decrease ofperformance in time).

The current invention aims at circumventing at least the mentioneddrawbacks of the known devices.

In addition, the invention has as an object to provide new LED deviceswith wavelength converting elements containing Mn⁴⁺-activated fluoridecompounds which are less toxic and less sensitive towards humidenvironments.

A further object is providing a novel class of Mn⁴⁺-activated fluoridecompounds with attractive luminescent properties for use in LED devices,which should preferably provide the devices the possibility of producingwarm white light.

According to the present invention, these and other objects are achievedby providing a light emitting diode device comprising a light emittingdiode arranged on a substrate and a wavelength converting elementcontaining a Mn⁴⁺-activated fluoride compound as a luminescent material,wherein the Mn⁴⁺-activated fluoride compound has a garnet-type crystalstructure.

The invention is based on the insight gained by the inventors that thesensitivity towards humid environments of Mn⁴⁺-activated fluoridecompound with a garnet-type crystal structure is considerably less thanthe sensitivity towards humid environment of the known compoundsdescribed in WO 2009/012301. The described compounds do not have agarnet-type crystal structure. The inventors moreover believe that, inview of the chemically inert character of the new invented luminescentcompounds, their toxicity is low as compared with similar knowncompounds disclosed in said patent publication. These properties of theluminescent compounds make their application in LED devices moreattractive, both in the production and in the use of the devices.

Fluorine compounds having a garnet-type crystal structure can berepresented by the following general formula: {A₃}[B₂](C₃)F₁₂, in whichF stands for fluoride and in which A, B and C represent ions of metal ormetal-like elements. These three types of ions are positionedrespectively on the dodecahedral, the octahedral and the tetrahedralsites of the garnet crystal structure. Generally speaking, elements Aand C are monovalent (+) whereas element B is trivalent (3+). However,especially on the octahedral sites, substitutions with chargecompensations are possible, so that also combinations of a bivalent anda tetravalent metal ion on these sites can be found.

The presence of both Mn⁴⁺ and F⁻ ions in the garnet structure isbelieved to be essential for providing the interesting narrow band orline emission in the red spectral region of the electromagneticspectrum. This means the region between appr. 600 and appr. 660 nm.

A preferred embodiment of the LED device according to the presentinvention is characterized in that the Mn⁴⁺-activated fluoride phosphorcompound answers the formula{A₂}[B_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d), in which formula A standsfor at least one element selected from the series consisting of Na⁺ andK⁺ and B stands for at least one element selected from the seriesconsisting of Al³⁺, B³⁺, Sc³⁺, Fe³⁺, Cr³⁺, Ti⁴⁺ and In³⁺, and in whichformula x ranges between 0.02 and 0.2, y ranges between 0.0 (and incl.0.0) and 0.4 (i.e. 0.0≦y<0.4) and d ranges between 0 (and incl. 0) and 1(i.e. 0≦d<1).

Although the above-mentioned inventive insight in principle can beachieved with all possible Mn⁴⁺-activated fluoride compounds having agarnet-type crystal structure, especially compounds with Na⁺ and/or K⁺on the dodecahedral sites, Al³⁺, B³⁺, Sc³⁺, Fe³⁺, Cr³⁺, Ti⁴⁺ and/orIn³⁺on the octahedral sites and Li⁺ on the tetrahedral sites arepreferred. Based on ion-radii considerations in combination withrequirements posed by the spatial structure of garnets, these preferredcompounds are believed to form highly stable crystalline compounds.

The Mn⁴⁺ ions are believed to be located on octahedral sites of thegarnet crystal structure. Ion radii calculations show that Mg²⁺ ispreferably present on the same crystal sites for charge compensationreasons. The amount of Mn⁴⁺ in the preferred compounds ranges between 1and 10 mol % based on the total B³⁺-ion content. A higher amount of Mn⁴⁺ions appears to cause a high so-called ‘self quenching’. If less than 1mol % Mn⁴⁺ is present on the octahedral sites of the garnet structure,no or hardly any activating effect is seen in the LED device. In suchmaterials, the absorption on Mn⁴⁺ appears to be negligible. Mn⁴⁺-amountsbetween 5 and 8 mol % are preferred, as in these conditions an optimalmatch between both the self-quenching effect and the desired absorbancelevel is reached.

In the preferred embodiment of the LED device, Mg²⁺ is also present onthe octahedral sites in the garnet structure. The presence of Mn⁴⁺causes charge imbalance in the garnet structure, which can becompensated by the presence of Mg²⁺. The amount of Mg²⁺ can be chosensomewhat broader as the amount of Mn⁴⁺. Therefore the amount of Mg²⁺ inthe preferred garnet compounds may range between 0 and 20 mol % based onthe total B³⁺-ion content, whereby the range includes the value 0 mol %.A higher amount of Mg²⁺ ions appears to cause the negative effect oflattice defects, e.g. anion vacancies. Mg²⁺-amounts between 1 and 10 mol% are preferred, as in these conditions an optimal match between bothcharge compensation and luminescence efficiency is reached.

Practice has shown that the amount of F can somewhat deviate from thestoichiometrical amount of 12 atoms per crystal cell unit. Thisdeviation is indicated by the factor d, which ranges between 0 (andincl. 0) and 1. It is stressed that, due to charge compensation effects,a small amount of the F can also be replaced by oxygen. This can be thecase if a small part of the trivalent ions of the octahedral sites arereplaced by ions of higher valence, like Ti⁴⁺. Under usual conditions,this will always be below appr. 8 mol % and is preferably below 4 mol %,all based on the total amount of F in the garnet structure. An increasein the amount of O²⁻ at the expense of F in the garnet structure mostgenerally causes an increased shift of the emission of the phosphorcompound into the deeper red, which is undesired.

A more preferred embodiment of the LED device according to the presentinvention is characterized in that in that the composition of theMn⁴⁺-activated fluoride compound substantially answers the formula{Na₃}[Al_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d). The ranges of theindices are as described before. From experimental data, it wasconcluded that, within the described broader class of garnet-typecompounds, this series of compounds is extremely stable. This stabilitymakes the application of these compounds in LED devices very attractive,both in the production and in the use of the devices.

A further interesting embodiment of the LED device according to theinvention has the feature that the wavelength converting element isformed as a ceramic platelet. This feature has especially value in LEDdevices to be used for producing white light. In principle theluminescent material can be formed with or without additional fillermaterials by pressing the materials to a sheet, sintering these sheetsaccording to a certain heating procedure and separating platelets ofdesired dimensions from said sintered sheet, for example by (laser)carving and breaking. As in this manner ceramic platelets of precisethickness can be manufactured, wavelength converting elements formed ofsuch platelets are very suitable in LED devices which should convert(near)UV or blue LED light into white light.

Another interesting embodiment of the LED device according to theinvention has the feature that the wavelength converting element isformed as a shaped body of resin material in which an amount of theMn⁴⁺-activated fluoride compound is incorporated. Said shaped body canfor example be formed as a lens or as a plate. However, other structuresare also possible within the scope of the invention. The amount offluoride compound with garnet-type crystal structure in the resin can bechosen dependent on the desired amount of converted light, the volume ofthe body, etc.

The invention also provides a new luminescent material containing aMn⁴⁺-activated fluoride compound. This material is characterized in thatthe compound has a garnet-type crystal structure. Materials of thiscomposition are relatively less toxic, have relatively low sensitivitytowards humid environments and show interesting emission spectra in thenear red region of the electromagnetic spectrum (600-660 nm).

Especially interesting is the material that answers the formula{A₃}[B_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d), in which formula A standsfor at least one element selected from the series consisting of Na⁺ andK⁺ and B stands for at least one element selected from the seriesconsisting of Al³⁺, B³⁺, Sc³⁺, Fe³⁺, Cr³⁺, Ti⁴⁺ and In³⁺, and in whichformula x ranges between 0.02 and 0.2, y ranges between 0.0 (and incl.0) and 0.4 and d ranges between 0 (and incl. 0) and 1. This material canbe advantageously applied in phosphor-coated LED devices. This holdsespecially for the luminescent material the composition of whichsubstantially answers the formula{Na₃}[Al_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d). For reasons describedbefore, luminescent materials wherein the amount of Mn⁴⁺ is between 1and 10 mol % whereas the amount of Mg²⁺ is between 1 and 20 mol % arepreferred. Most preferred however are compositions with a Mn⁴⁺-contentbetween 5 and 8.0 mol % and an Mg²⁺-content between 1 and 10 mol %.

Another interesting aspect of the invention relates to a method forpreparing a luminescent material as described in the previous paragraph.This method is characterized in that it encompasses the following steps:

Preparing a first aqueous solution by dissolving K₂MnF₆ in watercontaining at least 20 vol % HF,

Preparing a second aqueous solution of salts of the remaining metals ofwhich the intended garnet is composed, in molar ratio's corresponding tothe garnet composition,

Mixing stoichiometric amounts of both solution while stirring theresulting mixture, and

Isolating the resulting garnet composition from the mixture.

It will be clear to the skilled persons that the sequence in which thefirst and second aqueous solution are prepared is of no importance. Itis however highly preferred that, during the mixing of these solutions,the second solution is added to the first solution during stirring theso formed mixture. Care should be taken that the amount of added secondsolution is chosen so that a stoichiometric amount of Mn⁴⁺ to the amountof the other metals, which are already in stoichiometric amountsavailable in the first solution.

It is preferred that the first aqueous solution contains a small amountof NaHF₂. Adding this compound prevents that part of the Mn⁴⁺ isreduced. After mixing the solutions and stirring the mixture for some 5minutes, the resulting turbid solution is filtered off and washedseveral times with 2-propanol. The obtained powder is subsequently driedunder vacuum at 110° C. In order to obtain the right grain size, thepowder may be mechanically ground in a mortar. The so-obtained powder isanalyzed by X-ray and further used in wavelength converting elements ofLED devices according to the present invention.

It is stressed that not only the invented Mn⁴⁺-activated fluoridecompounds with garnet-type crystal structure in their pure form enhancethe desired performance of the light in a LED device, but that alsocomposite materials and mixed crystals of the invented compounds werefound to do so. Composites are defined as consisting of two or more onfinite scale distinguishable materials, e.g. core shell materials,composite ceramics or coated particles. A mixed crystal in contrast hasa homogenous distribution of the constituting elements on atomic scale.

This invention therefore also pertains to composites of{A₃}[B_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d) type garnets with oxidegarnets A₃B₂(CO₄)₃ including but not limited to YAG (Y₃Al₅O₁₂),Mg₃Al₂Si₃O₁₂ or Ca₃Al₂Si₃O₁₂. These composites are preferably oxidegarnet coatings on {Na₃}[Al_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d) typephosphor particles, or core shell materials, where the{Na₃}[Al_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d) type is surrounded by anoxide garnet shell. The difference between a coating and a shell ismainly the relative amount of the respective materials, whereas acoating is less than 10% w/w of the total material, in a core shellmaterial the shell may be 50% w/w or even more. The advantage of suchcoated or core shell materials are the increased stability with respectto humidity and the option to vary the refractive index of the phosphor.With increased stability it is also expected that toxicity will befurther reduced.

The same advantages are expected for mixed crystals of{Na₃}[Al_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d) with oxide garnets, withthe general formula of the mixed crystals: (1-a){Na₃}[Al_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d)*a A₃B₂(CO₄)₃. Theformation of such mixed crystals was found to enable the variation ofexcitation and emission wavelengths maxima and influence thermalquenching properties.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained and illustrated in terms of a number ofembodiments, with the help of the drawing, in which

FIG. 1 shows a first embodiment of the LED device according to theinvention,

FIG. 2 shows a second embodiment of the LED device according to theinvention,

FIG. 3 shows a graph of the emission spectrum of the first embodimentaccording to the invention,

FIG. 4 shows a graph of the emission spectrum of the second embodimentaccording to the invention, and

FIG. 5 shows a graph of the x-ray pattern of a sample of the inventedcompound {Na₃}[Al_(1.94)Mn_(0.03)Mg_(0.03)](Li₃)F₁₂ having a garnet-typecrystal structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention is schematically illustratedby FIG. 1. This Figure shows a cross-section of a LED device comprisinga semiconductor light emitting diode (1), which is connected to asubstrate (2), sometimes referred to as sub-mount. The diode (1) andsubstrate (2) are connected by means of appropriate connecting means,like solder or (metal-filled) adhesive.

The diode (1) is of the GaInN type, emitting during operation lighthaving a wavelength of 450 nm. In the present embodiment, said lightexits LED (1) via emitting surface (4). A wavelength converting element(3) formed as a convex lens shaped body is positioned adjacent to LED(1). This lens is largely made of a high temperature resistant siliconeresin, in which grains are incorporated of a Mn⁴⁺-activated fluoridecompound having a garnet-type crystal structure. Latter compound acts asa luminescent material in the lens. In the present embodiment saidsilicone resin contains 16 vol %{Na₃}[Al_(1.94)Mn_(0.03)Mg_(0.03)](Li₃)F₁₂, having a grain size of appr.10 micron. The type of silicone is chosen so that its refractive indexis almost identical with the refractive index of the phosphor compound,namely 1.34. By using (almost) identical refractive indices, scatteringlosses of the LED light through the wavelength converting element (3)are as low as possible.

In an alternative embodiment, the invented luminescent material iscompounded with highly transparent fluoroplastics (e.g. 3M Dyneon™THV2030G or THV220) with matched refractive index. The resultingcomposite may be transferred into a suitable shape by known techniques.These shapes may be used as functional optical parts of the LED orsimply as components for color conversion only.

The amount of luminescent compound and the dimensions of the wavelengthconverting element (3) are chosen so that all the blue light generatedby the LED (1) is converted into red light having a wavelength of appr.630 nm. A typical emission spectrum of the light exiting the heredescribed LED device is shown in FIG. 3. In this Figure, the intensityof the emission I (arbitrary units) is measured as a function of thewavelength λ (nm). It is stressed that, for adapting the color of theexiting red LED light, additional phosphors of other (known) types canbe used. Thus, the invention is not limited to LED devices comprisingonly a single phosphor of garnet-type crystal structure in thewavelength converting element (3), but mixtures of this phosphor withother (known) phosphors can be applied as well.

FIG. 2 depicts a schematic cross-section of a second embodiment of thepresent invention designed as a white light generating LED device. ThisFigure shows a conventional blue or (near) UV generating light emittingdiode (11), which is attached to a substrate (12) using solder bumps(not shown). Substrate (12) has metal contact pads on its surface towhich LED (11) is electrically connected (not shown). By means of thesesolder pads, LED (11) can be connected to a power supply. In the presentexample, LED (11) is of the AlInGaN type and emits blue light having apeak wavelength of appr. 420-470 nm. It goes without saying that othersemiconductor materials having other peak wavelengths can be used aswell within the scope of the present invention.

Two wavelength converting elements formed as ceramic platelets (13) and(14) are positioned adjacent to LED (11). The platelets (13, 14) and LED(11) can mutually be affixed by means of an adhesive (like a hightemperature resistant silicone material or a low melting glass) or bymeans of mechanical clamping. In the present embodiment, an adhesive isused. To keep unwanted absorptions as low as possible, the adhesivelayers between LED (11) and element (13) as well as between element (13)and element (14) have been made as thin as possible.

In the present embodiment, element (13) is shaped as a red phosphorplate whereas element (14) is shaped as a yellow phosphor plate. Thesurface dimensions of both plates are almost the same as the surfacedimension of the light emitting surface (15) of LED (11), although theymay be somewhat larger without having significant effect on the (white)exiting light. In case LED (11) is small enough, side emission of theblue radiation from the LED (11) can be ignored. The thicknesses of bothelements are typically in the range of 50-300 micron. The actualthickness of the platelets of course depends on the spectral powerdistribution of the LED light and the type of phosphor compound presentin the platelets.

In the described embodiment, the red phosphor platelet of element (13)was prepared of a pure Mn⁴⁺-activated fluoride phosphor compound with agarnet-type crystal structure. For this purpose the phosphor compoundsubstantially answered the formula{Na₃}[Al_(1.94)Mn_(0.03)Mg_(0.03)](Li₃)F₁₂ having a garnet-type crystalstructure. For the yellow phosphor platelet of element (14), thecompound Y₃Al₅O₁₂:Ce (‘Ce-doped YAG’) was used.

On the LED (11) and both wavelength conversion elements (13, 14) anoptical element (16) in the form of lens structure is placed, allowingoptimization of the emission pattern of the LED device. By means of aproper choice of this optical element, a Lambertian pattern can beobtained, but also a pattern that allows a good coupling with an opticalwaveguide structure. It is also possible to design the optical element(16) in such a way that a uniform illumination distribution of thegenerated white light is obtained. This makes the present LED devicevery suitable for backlighting in LCD type applications.

FIG. 4 shows a typical emission spectrum of the described white lightgenerating LED device according to FIG. 2. In this Figure, the intensityof the emission I (arbitrary units) is measured as a function of thewavelength λ (nm). The spectrum shows emission in the red spectralregion from appr. 600-appr. 660 nm, with an emission maximum around 630nm.

The luminescent material used in the wavelength converting element (3,13) of the LED devices as described above substantially answers theformula {Na₃}[Al_(1.94)Mn_(0.03)Mg_(0.03)](Li₃)F₁₂ and has a garnet-typecrystal structure . Said material was obtained as co-precipitates atroom temperature from aqueous HF solution containing Mn⁴⁺ as a dopant.For the preparation of said {Na₃}[Al_(1.94)Mn_(0.03)Mg_(0.03)](Li₃)F₁₂,stoichiometric amounts of the starting materials NaCl, LiCl, MgCl₂*6H₂Oand AlCl₃*6H₂O as well as a small amount of NaHF₂ were dissolved inwater and subsequently added to a 48% HF aqueous solution containingK₂MnF₆.The concentration of Mn⁴⁺ in the HF solution was 1 mol. %. Theprecipitates were filtered, washed repeatedly with 2-propanol, and thendried at 110° C. in vacuum. The obtained product was ground in a mortar.

FIG. 5 shows an X-ray powder pattern spectrum measured on arepresentative sample of one of the precipitates, using Cu-Kα radiation.In this Figure, the number of counts (N) is shown as a function of thediffracted angle 2Theta. With this measurement, these samples could beidentified to be {Na₃}[Al_(1.94)Mn_(0.03)Mg_(0.03)](Li₃)F₁₂ having agarnet-type crystal structure. No extra phases were detected in thissample.

It is stressed that it is possible to use a variety of other startingmaterials to produce the inventive garnet-type fluoride phosphors viaco-precipitation from aqueous solution. Especially hydroxides, nitrates,alkoxides, and carbonates are other good starting materials for use inthe co-precipitation method. Also other metal ion salts can be used asstarting material, like with salts of K⁺, B³⁺, Sc³⁺, Fe³⁺, Cr³⁺, Ti⁴⁺and/or In³⁺. When using these starting materials, Mn⁴⁺-activatedfluoride phosphor compound with garnet-type crystal structure of othercompositions can be prepared as well.

An amount of the {Na₃}[Al_(1.94)Mn_(0.03)Mg_(0.03)](Li₃)F₁₂ powderprepared as described above underwent further intense mechanicallygrinding until the mean particle size was appr. 5 micron. Subsequentlythe powder was pressed to a plate and sintered at 200° C. in a furnaceunder an axial pressure of 2 kbar. After cooling to room temperature,the so-obtained ceramic plate was scored with a laser and broken intoindividual platelets. These platelets were used as wavelength conversionelements in LED devices according to the present invention.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments. For example, itis possible to operate the invention in an embodiment wherein other(optical) element(s) are present between the LED and the wavelengthconverting elements or wherein more than one LED is operated incombination with one converting element.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. In the claims, the word “comprising” does not excludeother elements or steps, and the indefinite article “a” or “an” does notexclude a plurality. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measured cannot be used to advantage. Any reference signs inthe claims should not be construed as limiting the scope.

1. A light emitting diode device comprising: a light emitting diode (1,11) arranged on a substrate (2, 12), and a wavelength converting element(3, 13, 14) containing a Mn⁴⁺-activated fluoride compound as aluminescent material, wherein the Mn⁴⁺-activated fluoride compound has agarnet-type crystal structure.
 2. A light emitting diode deviceaccording to claim 1, wherein the Mn⁴⁺-activated fluoride compoundanswers the formula {A₃}[B_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d), inwhich formula A stands for at least one element selected from the seriesconsisting of Na⁺ and K⁺ and B stands for at least one element selectedfrom the series consisting of Al³⁺, B³⁺, Sc³⁺, Fe³⁺, Cr³⁺, Ti⁴⁺ andIn³⁺, and in which formula 0.02<x<0.2, 0.0≦y<0.4, and 0≦d<1.
 3. A lightemitting diode device according to claim 2, wherein the composition ofthe Mn⁴⁺-activated fluoride compound material substantially answers theformula {Na₃} [Al_(2— x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d).
 4. A lightemitting diode device according to claim 1, 2 or 3, wherein thewavelength converting element (3, 13) is formed as a ceramic platelet.5. A light emitting diode device according to claim 1, wherein thewavelength converting element (3, 13) is formed as a shaped body ofresin material in which an amount of the Mn⁴⁺-activated fluoridecompound is incorporated.
 6. A luminescent material containing aMn⁴⁺-activated fluoride compound, wherein the compound has a garnet-typecrystal structure.
 7. A luminescent material according to claim 6,wherein the composition answers the formula{A₃}[B_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d), in which formula A standsfor at least one element selected from the series consisting of Na⁺ andK⁺ and B stands for at least one element selected from the seriesconsisting of Al³⁺, B³⁺, Sc³⁺, Fe³⁺, Cr³⁺, Ti⁴⁺and In³⁺, and in whichformula 0.02<x<0.2, 0.0≦y<0.4, and 0≦d<1.
 8. A luminescent materialaccording to claim 7, wherein the composition of the Mn⁴⁺-activatedfluoride compound substantially answers the formula{Na₃}[Al_(2-x-y)Mn_(x)Mg_(y)](Li₃)F_(12-d)O_(d).
 9. A method forpreparing a luminescent material having a composition according to claim6, characterized in that it encompasses the following steps: Preparing afirst aqueous solution by dissolving K₂MnF₆ in water containing at least20 vol % HF, Preparing a second aqueous solution of salts of theremaining metals of which the intended garnet is composed, in molarratio's corresponding to the garnet composition, Mixing stoichiometricamounts of both solution while stirring the resulting mixture, andIsolating the resulting garnet composition from the mixture.
 10. Amethod for preparation a luminescent material according to claim 8,wherein the first aqueous solution contains NaHF₂.